1. Field of the Invention
The present invention relates to an infrared sensor and an electronic device including the same, and, more particularly, to an infrared sensor used in, for example, an aural thermometer and an electronic device including the same.
2. Description of the Related Art
FIG. 10 is a partially exploded perspective view of a related infrared sensor using a thermopile element. FIG. 11 is a schematic longitudinal sectional view of FIG. 10. In FIGS. 10 and 11, an infrared sensor 1 includes a metallic stem 2, a thermopile element 4, a chip-type thermistor 5 serving as a thermo-sensing element, and a metallic case 6.
Lead terminals 3 are provided at the stem 2 via glass sealing. One end (inner lead) of each lead terminal 3 is exposed at a component-mounting surface of the stem 2, while the other end (outer lead) of each lead terminal 3 protrudes towards the opposite side. The thermopile element 4 is bonded to substantially the central portion of the component-mounting surface of the stem 2 with, for example, an adhesive having good thermal conductivity, and is connected to the top portions of some of the inner leads via wires. The thermistor 5 is adjacent to the thermopile element 4 and is mounted on the stem 2, and is connected to the top portion of one of the inner leads via a wire. In general, since the central portion of the thermopile element 4 is a hot junction and a portion near the peripheral portion of the thermopile element 4 is a cold junction, the thermistor 5 is substantially disposed near the cold junction of the thermopile element 4. An edge of the case 6 is secured to and provided along the periphery of the stem 2 so as to cover the thermopile element 4, the thermistor 5, and the inner leads of the lead terminals 3. An infrared ray transmitting window 7 is provided at the top surface of the case 6 such that infrared rays emitted from a detection object are incident upon the hot junction of the thermopile element 4. For example, a silicon (Si) or germanium (Ge) material having an infrared ray interference filter for selectively passing infrared rays is used for the infrared ray transmitting window 7.
In the infrared sensor 1 having such a structure, incident infrared rays transmitted through the infrared ray transmitting window 7 from the outside are incident upon the hot junction of the thermopile element 4. In FIG. 11, reference character a represents an infrared ray incident upon the infrared sensor 1 from the outside, and reference character b represents an infrared ray transmitted through the infrared ray transmitting window 7 and incident upon the thermopile element 4. The temperature of the hot junction of the thermopile element 4 increases due to the incident infrared rays, resulting in a difference between the temperatures of the hot junction and the cold junction. In accordance with the temperature difference, an electromotive force is produced and output. Since the thermistor 5 is disposed near the cold junction of the thermopile element 4, its temperature is substantially the same as the temperature of the cold junction. Therefore, the thermistor 5 detects and outputs the absolute temperature of the cold junction. Here, the term “absolute temperature” is used to refer to a relative temperature difference, and does not refer to a temperature in the Kelvin temperature scale. Using an output corresponding to the difference between the temperature of the cold junction detected by the thermistor 5 and the temperature output from the thermopile element 4, the temperature of a detection object is measured.
Since the case 6 is made of a metal, it generally does not allow infrared rays to pass. However, although an edge of the case 6 is connected to the stem 2, the further away a portion of the case 6 is from the connection portion with the stem 2, the more difficult it is to maintain it at the same temperature as the stem 2. Therefore, the temperature of the case 6 increases due to convection of outside air, irradiation of the outside surface of the case 6 with infrared rays, or direct heating of the case 6.
When the temperature of the case 6 increases, a difference between the temperatures of the case 6 and the stem 2 occurs. When the temperature difference occurs, infrared rays are radiated to the stem 2 from the case 6 via secondary emission. In FIG. 11, the infrared rays that are radiated from the case 6 by secondary emission are represented by reference characters c. Since these infrared rays produced by secondary emission are also radiated towards the thermopile element 4 on the stem 2 from an inside surface of the case 6, the temperature of the hot junction of the thermopile element 4 is increased not only by the infrared rays that enter the infrared sensor 1 through the infrared ray transmitting window 7, but also by infrared rays that are produced by secondary emission from the case 6. Therefore, the temperature of a detection object can no longer be accurately measured.
The fact that the difference between the temperatures of the case 6 and the stem 2 affects temperature measurements means that, when the temperature of the case 6 is temporarily changed due to the aforementioned reason and reasons other than that mentioned above, the temperature detected by the infrared sensor 1 also temporarily changes. This is because a change in temperature of either one of the cold junction of the thermopile element 4 and the thermistor 5 due to a change in the temperature of the case 6 does not correspond to a change in temperature of the other of the cold junction of the thermopile element 4 and the thermistor 5. Accordingly, in the related infrared sensor 1, the detected temperature of a detection object is substantially affected by a temporary change in the temperature (that is, a disturbance) of anything other than the detection object.
To overcome this problem, for example, Japanese Unexamined Patent Application Publication No. 8-101062 discloses a structure including a shield tube for covering an infrared detector in order to prevent detection of secondary emission from a package. In this case, an error resulting from detection of secondary emission is decreased.
However, even in this case, since the infrared detector is mounted on the electronic cooling device, the shield tube is disposed so as to surround the electronic cooling device, such that it is difficult to make the temperature of the infrared detector and the temperature of the shield tube the same. In particular, the infrared detector is merely mounted on the top surface of the electronic cooling device, such that, by, for example, convection of the air above, the temperature of the infrared detector may be different from the temperature of the electronic cooling device, and, thus, from the temperature of the shield tube. If there is a difference between the temperatures of the infrared detector and the shield tube, secondary emission is not restricted. Therefore, an error resulting from detection of secondary emission is not satisfactorily reduced. In addition, the problem of the detected temperature of a detection object being easily affected by a temporary change in the temperature of anything other than the detection object is not satisfactorily prevented.